Measuring assemblies of this type are provided with measuring cells or measuring sensors, e.g. electrochemical gas sensors, that are used to study gaseous media, in particular toxic and non-toxic gases. Usually, the measuring cells are attached by means of releasable screw connections to the gas-conducting channels or measuring chambers. These types of attachments, which use screws distributed over the circumference of the measuring cell, result in the following problems that are particularly critical, due to the fact that the measuring cells must be replaced after a certain operating time or in case of a defect.
If the screws are not uniformly tightened when attaching the measuring cell, an uneven surface pressure may result, possibly causing leakages in the measuring system. Since air entering through a leak reaches the measuring cells via these leakages, the measuring result is distorted.
The uneven pressure furthermore leads to distortions of the measuring cell or sensor housing, possibly resulting in tears in the material. These tears permit air entering through leakages to reach the measuring system, or alternatively, liquid may drain from the interior of the measuring cell. Both defects lead to distortions of the measuring value and, ultimately, to the unfitness of the measuring cell.
These disadvantages can be avoided only if the attachment screws are tightened to comply with a maximum torque value by using a torque wrench, which is not generally available to the user. Furthermore, during a measuring cell replacement, care must be taken that the attachment screws be tightened diagonally, in order to attain a uniform distribution of sealing pressure.
Another disadvantage is that the effects of mechanical force on the measuring cell housing or on the circuit board of the evaluation circuit, if the latter is mechanically connected to the measuring cell housing, lead to distortions in the measuring values. This means, however, that following a measuring cell replacement, the measuring system must again be adjusted. This usually requires a test gas that is frequently not available to the user.
In practice, this problem is solved by the measuring cell being adjusted at the factory and labeled with a corresponding coefficient that can be used to set the evaluation circuit of the respective measuring cell after installation has been completed, without the application of test gas. But such an adjustment presupposes identical and reproducible installation conditions in the measuring cell, equal to those coefficients determined at the factory. If this is not ensured, measuring errors will again occur.
To avoid these errors, two important boundary conditions must be met. On one hand, the assembly must be constructed so that the measuring cell can be mounted in a reproducible manner, always under the same conditions. On the other hand, the evaluation and processing electronics must be connected to the measuring cell in such a way that no later adjustment by the user is required.
It is indeed known that the circuit boards with a signal evaluation circuit are supplied with the measuring cell to which they are electrically connected. But if a secure connection between the measuring cell and circuit board is not made, vibrations during insertion may result in contact problems, and thus in measuring errors. Screwing the circuit board to the measuring cell causes problems, since mechanical bending strains may lead to tearing and/or measuring errors.
The invention at hand is thus based on the task of creating a measuring assembly that enables a problem-free exchange of the adjusted measuring cell with the signal evaluation circuit, and Which is constructed to prevent installation errors leading directly or indirectly to measuring value distortions.